Juncus acutus is a prickly, erect perennial rush <1,5m high in the monocot family Juncaceae. It originates from Europe, Africa, America and Macronesia. It grows in all kinds of soils such as sandy sea shores, dune slacks, coastal flats, wet areas like lowland grassland, grassy woodland, riparian vegetation, freshwater wetland, saline and sub saline wetlands. In some countries including New Zealand and Australia it is considered to be an invasive weed. Juncus acutus displaces native rushes & sedges. The recreational quality of the habitat is greatly reduced due to its sharp leaves and stems. It is a problem in coastal areas of Northland, Auckland, Waikato and the Manawatu.

Juncus acutus forms a stout, dense, prickly clump. The pith filled flower stems are round in shape, smooth and are a light green in colour and end in a very sharp point. The leaves which look similar to the stems arise from the base at different angles giving the plant a globe shape. The leaves also end with a stiff sharp point. These can be harmful to young children especially their eyes.

The flower stems emerge from the base at all angles and each has a dense flower cluster near the end of each stem. Each flower has 6 stamens. The flowers are hermaphrodite and are pollinated by the wind. The fruits are oval 3-celled brown capsules of 4 millimetres to 6 millimetres. The 1.2 millimetres to 2 millimetres long brown seeds have a tail at each end. The seeds are spread by water and the plant can spread locally by rhizomes.